Can manufacture

ABSTRACT

A method and apparatus are disclosed which are suitable for use in the manufacture of two-piece metal containers. In particular, a press is disclosed which makes cup sections from metal sheet using a combination of drawing and stretching operations. The cups resulting from the press have the advantage of having a base thickness that is thinner relative to the ingoing gauge of the metal sheet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application No.PCT/EP2011/055847, filed Apr. 13, 2011, which claims priority toEuropean Application No. 10159826.6, filed Apr. 13, 2010, the contentsof each of which are incorporated by reference in their entireties.

TECHNICAL FIELD

This invention relates to the production of metal cups and in particular(but without limitation) to metal cups suitable for the production of“two-piece” metal containers.

BACKGROUND

U.S. Pat. No. 4,095,544 (NATIONAL STEEL CORPORATION) Jun. 20, 1978details conventional Draw & Wall Ironing (DWI) and Draw & Re-Draw (DRD)processes for manufacturing cup-sections for use in making two-piecemetal containers. [Note that in the United States of America, DWI isinstead commonly referred to as D&I.] The term “two-piece” refers to i)the cup-section and ii) the closure that would be subsequently fastenedto the open end of the cup-section to form the container.

In a DWI (D&I) process (as illustrated in FIGS. 6 to 10 of U.S. Pat. No.4,095,544), a flat (typically) circular blank stamped out from a roll ofmetal sheet is drawn though a drawing die, under the action of a punch,to form a shallow first stage cup. This initial drawing stage does notresult in any intentional thinning of the blank. Thereafter, the cup,which is typically mounted on the end face of a close fitting punch orram, is pushed through one or more annular wall-ironing dies for thepurpose of effecting a reduction in thickness of the sidewall of thecup, thereby resulting in an elongation in the sidewall of the cup. Byitself, the ironing process will not result in any change in the nominaldiameter of the first stage cup.

FIG. 1 shows the distribution of metal in a container body resultingfrom a conventional DWI (D&I) process. FIG. 1 is illustrative only, andis not intended to be precisely to scale. Three regions are indicated inFIG. 1:

-   -   Region 1 represents the un-ironed material of the base. This        remains approximately the same thickness as the ingoing gauge of        the blank, i.e. it is not affected by the separate manufacturing        operations of a conventional DWI process.    -   Region 2 represents the ironed mid-section of the sidewall. Its        thickness (and thereby the amount of ironing required) is        determined by the performance required for the container body.    -   Region 3 represents the ironed top-section of the sidewall.        Typically in can making, this ironed top-section is around        50-75% of the thickness of the ingoing gauge.

In a DRD process (as illustrated in FIGS. 1 to 5 of U.S. Pat. No.4,095,544), the same drawing technique is used to form the first stagecup. However, rather than employing an ironing process, the first stagecup is then subjected to one or more re-drawing operations which act toprogressively reduce the diameter of the cup and thereby elongate thesidewall of the cup. By themselves, most conventional re-drawingoperations are not intended to result in any change in thickness of thecup material. However, taking the example of container bodiesmanufactured from a typical DRD process, in practice there is typicallysome thickening at the top of the finished container body (of the orderof 10% or more). This thickening is a natural effect of the re-drawingprocess and is explained by the compressive effect on the material whenre-drawing from a cup of large diameter to one of smaller diameter.

Note that there are alternative known DRD processes which achieve athickness reduction in the sidewall of the cup through use of small orcompound radii draw dies to thin the sidewall by stretching in the drawand re-draw stages.

Alternatively, a combination of ironing and re-drawing may be used onthe first stage cup, which thereby reduces both the cup's diameter andsidewall thickness. For example, in the field of the manufacture oftwo-piece metal containers (cans), the container body is typically madeby drawing a blank into a first stage cup and subjecting the cup to anumber of re-drawing operations until arriving at a container body ofthe desired nominal diameter, then followed by ironing the sidewall toprovide the desired sidewall thickness and height.

However, DWI (D&I) and DRD processes employed on a large commercialscale have a serious limitation in that they do not act to reduce thethickness (and therefore weight) of material in the base of the cup. Inparticular, drawing does not result in reduction in thickness of theobject being drawn, and ironing only acts on the sidewalls of the cup.Essentially, for known DWI (D&I) and DRD processes for the manufactureof cups for two-piece containers, the thickness of the base remainsbroadly unchanged from that of the ingoing gauge of the blank. This canresult in the base being far thicker than required for performancepurposes.

The metal packaging industry is fiercely competitive, with weightreduction being a primary objective because it reduces transportationand raw material costs. By way of example, around 65% of the costs ofmanufacturing a typical two-piece metal food container derive from rawmaterial costs.

There is therefore a need for improved light-weighting of metalcup-sections in a cost-effective manner. Note that in this document, theterms “cup-section” and “cup” are used interchangeably.

SUMMARY

Accordingly, in a first aspect of the invention (defined in claim 1)there is provided a method for manufacture of a metal cup from a metalsheet, the method comprising the following operations:

-   -   i. a drawing operation comprising drawing the metal sheet into a        cup having a sidewall and an integral base;    -   ii. a stretching operation performed on the cup, the operation        comprising clamping an annular region on the base of the cup to        define an enclosed portion, and deforming and stretching at        least part of the base that lies within the enclosed portion to        thereby increase the surface area and reduce the thickness of        the base, the annular clamping adapted to restrict or prevent        metal flow from the clamped region into the enclosed portion        during this stretching operation;    -   the drawing and stretching operations performed in a common        press.

In a second aspect of the invention (defined in claim 5) there isprovided a press for manufacture of a metal cup from a metal sheet, thepress comprising:

-   -   i. means for drawing the metal sheet into a cup having a        sidewall and an integral base;    -   ii. a clamping element for clamping the drawn cup during a        stretching operation, the clamping element adapted to clamp an        annular region on the base of the drawn cup to define an        enclosed portion;    -   iii. a stretch tool adapted to deform and stretch at least part        of the base that lies within the enclosed portion in the        stretching operation to thereby increase the surface area and        reduce the thickness of the base, the clamping element further        adapted to restrict or prevent metal flow from the clamped        region into the enclosed portion during the stretching        operation.

The method and apparatus of the different aspects of the invention havethe advantage (over known processes/apparatus) of achieving manufactureof a cup having a base which is thinner than the ingoing gauge of themetal sheet, without requiring loss or waste of metal. This is achievedby use of a single press, thereby simplifying the manufacturing process.When applied to the manufacture of two-piece containers, the inventionenables cost savings to be made of the order of several dollars per1,000 containers relative to existing manufacturing techniques.

To ensure that the enclosed portion (and therefore the cup's base) isstretched and thinned during the stretching operation, the base of thedrawn cup is clamped sufficiently to restrict or prevent metal flow fromthe clamped region into the enclosed portion during the stretchingoperation. If the clamping loads are insufficient, material from theclamped region (or from outside of the clamped region) would merely bedrawn into the enclosed portion, rather than the enclosed portion (andthe cup's base) undergoing any thinning It has been found thatstretching and thinning can still occur when permitting a limited amountof flow of material from the clamped region (or from outside of theclamped region) into the enclosed portion, i.e. when metal flow isrestricted rather than completely prevented.

The method and apparatus of the invention is particularly suitable foruse in the manufacture of metal containers, with the final resulting cupbeing used for the container body. The final resulting cup may be formedinto a closed container by the fastening of a closure to the open end ofthe cup. For example, a metal can end may be seamed to the open end ofthe final resulting cup. However, typically the cup resulting from themethod of the invention would be subjected to either or a combination ofa re-drawing operation and an ironing operation. The re-drawingoperation may comprise one or more stages, each stage having the effectof inducing a staged reduction in cup diameter. The ironing operationwould have the benefit of increasing the height of the sidewall of thecup produced by the method and apparatus of the invention. Preferably,the stretching operation comprises deforming and stretching at leastpart of the base that lies within the enclosed portion into a domedprofile. In the field of metal containers for carbonated beverages, itis common for the base of the container body to be inwardly-domed toresist pressure generated by the product. The “dome” provided by themethod and apparatus of the invention may serve as the inwardly-domedregion of a beverage container body. However, it is likely that the cupwould undergo a later reforming operation to provide the domed base ofthe cup with a desired final profile necessary to resist in-canpressure.

The method of the invention is suitable for use on cups that are bothround and non-round in plan. However, it works best on round cups.

One other way of minimising the amount of material in the base ofcup-sections produced using conventional DWI and DRD processes would beto use thinner gauge starting stock. However, tinplate cost per tonneincreases as the gauge decreases. This increase is explained byadditional costs of rolling, cleaning and tinning the thinner steel.When also taking account of material usage during manufacture of atwo-piece container, the variation in net overall cost to manufacturethe container versus ingoing gauge of material looks like the graphshown in FIG. 2. This graph demonstrates that from a cost perspective,going for the thinnest gauge material does not necessarily reduce costs.In essence, there is a cheapest gauge of material for any container of agiven sidewall thickness. The graph also shows the effect of reducingthe thickness of the top and mid-wall sections of the container indriving down the cost curve. FIG. 3 shows the same graph based uponactual data for UK-supplied tinplate of the type commonly used incan-making. For the material illustrated in FIG. 3, 0.285 mm representsthe optimum thickness on cost grounds, with the use of thinner gaugematerial increasing net overall costs for can production. The graph ofFIG. 3 shows the percentage increase in overall cost per 1,000 cans whendeviating from the 0.285 mm optimum ingoing gauge thickness.

The final resulting cup of the invention has the benefits of a thinner(and therefore lighter) base.

The “metal sheet” can include a blank cut from a larger expanse of metalsheet.

By “annular clamping” or clamping an “annular region” is meant that thebase of the drawn cup is clamped either continuously or at spacedintervals in an annular manner.

The clamping element may be in the form of a continuous annular sleeve;alternatively, it may be a collection of discrete clamping elementsdistributed in an annular manner to act against the metal sheet.

The method and apparatus of the invention are not limited to aparticular metal. They are particularly suitable for use with any metalscommonly used in DWI (D&I) and DRD processes. Also, there is nolimitation on the end use of the cup that results from the method andapparatus of the invention. Without limitation, the cups may be used inthe manufacture of any type of container, whether for food, beverage oranything else.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a container body of the backgroundart resulting from a conventional DWI process. It shows the distributionof material in the base and sidewall regions of the container body.

FIG. 2 is a graph showing in general terms how the net overall cost ofmanufacturing a typical two-piece metal container varies with theingoing gauge of the sheet metal. The graph shows how reducing thethickness of the sidewall region (e.g. by ironing) has the effect ofdriving down the net overall cost.

FIG. 3 is a graph corresponding to FIG. 2, but based on actual pricedata for UK-supplied tinplate.

Embodiments of the invention are illustrated in the following drawings,with reference to the accompanying description:

FIG. 4 is a cross-section through a press of the invention showing ablank of metal sheet prior to the drawing and stretching operations.

FIG. 5 is a cross-section through the press of FIG. 4, but after thedrawing operation to draw the blank of metal sheet into a cup having asidewall and integral base.

FIG. 6 is a cross-section through the press of FIGS. 4 and 5, but afterthe stretching operation to deform and stretch the base of the drawncup.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Mode(s) for CarryingOut the Invention Drawing Operation

FIG. 4 shows a combined drawing and stretching press 10. A blank ofmetal sheet 5 is “slidably clamped” in position between the opposingsurfaces of a draw pad 11 and an end face of a draw die 12. A draw punch13 is located above the upper surface of the blank of metal sheet 5Within the bore defined by the draw die 12 is a stretch punch 14 whichis radially-inwards of and surrounded by annular clamping element 15.

Once the blank 5 is “slidably clamped” between the draw pad 11 and thedraw die 12, the draw punch 13 is moved axially downwards (along axis16) until the peripheral annular region 17 of the end face of the punchcontacts a corresponding region on the blank 5 (see FIG. 5). The drawpunch 13 is urged downwards through the bore of the draw die 12 toprogressively draw the initially planar blank 5 into a cup 30 having asidewall 31 and integral base 32.

By “slidably clamping” is meant that the clamping load during drawing isselected so as to permit the metal sheet 5 to slide, relative towhatever clamping means is used (in this case a “draw pad 11”), inresponse to the deforming action of the draw punch 13/draw die 12 on themetal sheet. An intention of this slidable clamping is to prevent orrestrict wrinkling of the material during drawing.

Stretching Operation

Once the draw punch 13 reaches the end of its stroke, annular clampingelement 15 is moved axially upwards until an annular region 33 on thebase 32 of the drawn cup is clamped between the annular clamping elementand the peripheral annular region 17 of the end face of the draw punch13 (see FIG. 5). The material of the cup's base enclosed by this annularclamping is referred to as the “enclosed portion” 34.

Stretch punch 14 is then moved axially upwards (along axis 16) tocontact the enclosed portion 34 (see FIG. 6). The stretch punch 14 isurged upwards into the recess 18 defined within the end face of the drawpunch 13 to progressively deform and stretch the material of theenclosed portion 34 of the cup 30 into a domed profile 35 (see FIG. 6).The clamping load applied between the annular clamping element 15 andthe peripheral annular region 17 of the end face of the draw punch 13 issufficient to restrict or prevent metal flow from the clamped annularregion 33 into the enclosed portion 34 during this stretching operation.To enhance the gripping effect, the end face of the annular clampingelement 15 may be textured (not shown in the figures) to thereby allowuse of a reduced clamping load compared to use of an annular clampingelement having a smooth polished end face.

As shown in FIG. 6, the cup that results from the stretching operationperformed in press 10 has a reduced thickness in its base relative tothe ingoing gauge of the blank of metal sheet 5. As indicated above inthe general description of the invention, this cup may be used as acontainer body, but would generally be subjected to either or acombination of redrawing and ironing operations to optimise the cupdiameter and sidewall thickness. As also indicated in the generaldescription of the invention, the domed region 35 may be particularlybeneficial for containers intended for pressurised products, such ascarbonated beverages.

What is claimed:
 1. A method for manufacture of a metal cup from a metalsheet, the method comprising the following operations: i. a drawingoperation comprising drawing the metal sheet into a cup having asidewall and an integral base; ii. a stretching operation performed onthe cup, the operation comprising clamping an annular region on the baseof the cup to define an enclosed portion, and deforming and stretchingat least part of the base that lies within the enclosed portion tothereby increase the surface area and reduce the thickness of the base,the annular clamping adapted to restrict or prevent metal flow from theclamped region into the enclosed portion during this stretchingoperation; the drawing and stretching operations performed in a commonpress.
 2. The method as claimed in claim 1, wherein the drawingoperation comprises locating the metal sheet between a draw punch and adraw die, and moving either or both of the draw punch and draw dietowards each other such that the draw punch extends through the draw dieto thereby draw the metal sheet into the cup, the annular clampingduring the stretching operation comprising clamping the annular regionon the base of the cup between the draw punch and a clamping elementdisposed on the opposite side of the cup to the draw punch.
 3. Themethod as claimed in claim 2, wherein the stretching operation comprisesusing a stretch punch disposed on the opposite side of the cup to thedraw punch and moving either or both of the stretch punch and clampedcup towards each other to deform and stretch at least part of the basethat lies within the enclosed portion.
 4. The method as claimed in claim1, wherein the stretching operation comprises deforming and stretchingat least part of the base that lies within the enclosed portion into adomed profile.
 5. A press for manufacture of a metal cup from a metalsheet, the press comprising: i. means for drawing the metal sheet into acup having a sidewall and an integral base; ii. a clamping element forclamping the drawn cup during a stretching operation, the clampingelement adapted to clamp an annular region on the base of the drawn cupto define an enclosed portion; iii. a stretch tool adapted to deform andstretch at least part of the base that lies within the enclosed portionin the stretching operation to thereby increase the surface area andreduce the thickness of the base, the clamping element further adaptedto restrict or prevent metal flow from the clamped region into theenclosed portion during the stretching operation.
 6. The press asclaimed in claim 5, wherein the means for drawing the metal sheetcomprises a draw punch and a draw die, either or both of the draw punchand draw die being moveable towards each other such that the draw punchmay extend through the draw die to draw the metal sheet into the cup,the clamping element disposed on the opposite side of the cup to thedraw punch such that in use during the stretching operation the clampingelement clamps the annular region of the base between the clampingelement and the draw punch.
 7. The press as claimed in claim 6, whereinthe stretch tool comprises a stretch punch disposed on the opposite sideof the cup to the draw punch, either or both the stretch punch andcombination of draw punch and clamping element being moveable towardseach other such that in use the stretch punch deforms and stretches atleast part of the base that lies within the enclosed portion.
 8. Thepress as claimed in either of claim 6, wherein the draw punch isprovided with a recess or bore to enable the stretch tool to extend allor partly within the recess or bore during the stretching operation. 9.A container comprising: a circumferential sidewall defining an end; anda bottom portion enclosing the end of the circumferential sidewall,wherein a material of the bottom portion is stretched relative to thecircumferential sidewall to form a thinner preselected profile.
 10. Thecontainer of claim 9, wherein the thinned preselected profile is a dome.11. The container of claim 10, wherein the material of the container ator about the dome has a substantially uniform thickness.
 12. Thecontainer of claim 10, wherein the container is formed from a blank ofmaterial; wherein the blank of material has a base gauge prior to beingformed; wherein, after being formed, the material of the container at orabout the dome has a thickness; and wherein the thickness of thematerial at or about the dome is less than the base gauge.
 13. Thecontainer of claim 12, wherein the thickness of the material at or aboutthe dome is about 0.0003 inch to about 0.003 inch thinner than the basegauge.
 14. The container of claim 9, wherein the container is formedfrom a blank of material; and wherein the blank of material has apreformed dome portion.
 15. The container of claim 9, wherein thecontainer is a can body.
 16. The container of claim 9, wherein thecontainer is a cup.